This statement isn’t entirely truthful. I don’t recall any kind of similar “borrowed power” during Wrath, Cata or even MoP. All that mattered was gear and your ability to perform with the class.
An argument could also be made, that while “classes being complete” hasn’t exactly ever been a reality, there was a time where all classes and specs were very close to viable and usable. Always will be meta, but it’s more recent that entire specs go utterly broken for years before being addressed.